Blood omen takes place about..400 years or so after the original blood omen. It follows the alternate timeline created by Soul reaver 2's ending, in which (SPOILER)Kain pulls the soul reaver out of Raziel before it can consume him. I would not advise you to play it untill you have completed Defiance, as parts of the story will make no since otherwise.

Defiance takes place in two timelines for the majority of the game. When you play as Kain, it takes place in the Sarafan crusades era and Raziel's takes place in the original blood omen era.

The games should be played in this order to have the story flow consistently:Blood omen-Soul reaver-Soul reaver 2-Defiance-Blood omen 2

You should only play Blood omen 2 if you are a hardcore fan and want to see how some (but far from all) of the story twist from Defiance get wrapped up.